In the petroleum industry, levels of sulfur in crude have increased steadily in recent years with the extraction of heavier crude, with some crudes containing up to 5 wt. % sulfur. Sulfur corrosion is a problem in a number of industries. In refineries, crudes and distillate fractions contain H2S and sulfur species such as mercaptans, disulfides, aliphatic sulfides, etc. (collectively, “sulfur”). It is believed that sulfur species decompose forming H2S, corroding equipment and/or causing severe damages to equipment including sulfide stress cracking (SSC), hydrogen induced cracking (HIC), stress oriented hydrogen induced cracking (SOHIC). Sulfur corrosion also affects the power industry in the burning of fossil fuels to generate energy, and in the operation of wet scrubbers to remove pollutants from furnace flue gas or other gas streams.
Industry task groups have been formed to provide guidelines to avoid corrosion. The Corrosion Society group NACE (“National Association of Corrosion Engineers”) published a document in 2004 (NACE 34103), the contents of which are incorporated by reference herein, indicating that lower than 5% Cr steels are not currently used in refineries to avoid sulfidic corrosion, 5Cr steels are used up to 343° C., and 9Cr steel are used up to 400° C. 300-series austenitic stainless steels, commonly known as “18-8” (roughly 18 Cr-8 Ni) alloys, are indicated to have a high degree of resistance to sulfidic corrosion.
After the 2004 publication of the Corrosion Society group NACE, the API Subcommittee on Corrosion and Materials also issued guidelines for avoiding sulfidation corrosion (API Recommend Practice 939-C Guidelines for Avoiding Sulfidation (Sulfidic) Corrosion Failures in Oil Refineries, First Edition), the contents of which are incorporated by reference herein. It is indicated that sulfidic corrosion rate increases with temperature from 230° C. until 425° C., at approximately which point the corrosion rate reaches a peak.
As in the NACE findings, the API Subcommittee found chromium to be a beneficial alloying element protecting against corrosion, e.g., the corrosion rate of steel with 5% Cr is lower than the corrosion rate of carbon steel. In general, the ranking of steels and alloys with respect to corrosion resistance, particularly sulfidation corrosion, ranges according to the following order from low to high: carbon steel (ASTM A53), carbon steel (ASTM A106), carbon steel plus 0.5% Mo; 5 Cr steel+0.5% Mo; and 300-series austenitic stainless steels.
In general, the Committee findings acknowledge an improvement in sulfidation resistance with the addition of Si of up to 0.25% to carbon steel containing no Si. It is indicated that the Si effect plateaus out after the amount is reached. There are other references disclosing the addition of higher Si amounts. US Patent Application No. 20110315276 discloses a low alloy steel with a high yield strength and excellent sulphide stress cracking (“SSC”) properties, for use in tubular products for hydrocarbon wells containing hydrogen sulphide (H2S). Si is added for deoxidation purpose. However, the reference teaches that “beyond 0.5%, it results in deterioration of SSC resistance. For this reason, its content is fixed to between 0.1% and 0.5%.”
In a 1987 paper, Cihal et al. conducted a study on sulphidic corrosion of austenitic corrosion resisting steel containing 20% Cr, 20% Ni and various levels of Si and P (0.09% to 5.4% Si). It was indicated that the highest resistance to sulphide corrosion was recorded for the steel with the low content of Si, P and S. Furthermore, “[t]he sensitivity to sulfide corrosion cracking increases with increasing silicon content.” (See “Sulphide Corrosion Cracking of Corrosion Resisting Steels with Various Silicon Content,” Kovove Materialy, Vol. 27, 1987, No. 4, pp. 399-407).
There is still a need for a new class of materials for equipment subject to corrosion and stress cracking in sulfur containing environments.